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Definition of "rectangular-diaphragm":

Although most condenser microphones employ circular capsules, some use alternative shapes, such as ellipses, “ears,” or rectangles.

The rectangular capsule was developed by Rune Rosander of Pearl Microphone Labs in the 1950s. The company split in the 1970s, but the trademark capsule design was shared by the two descendents, Pearl Microphone Lab and Milab Microphones.

Because a condenser microphone’s self-noise level in general diminishes with size, rectangular diaphragms of these proportions match the low self-noise levels of traditional circular large-diaphragm capsules.

According to PML and Milab, rectangular capsules have advantages over circular designs.

One is that the diaphragm’s natural resonance, which is a function of the length of the membrane, is divided into two frequencies — one for the length, and one for the width. In contrast, a round membrane has a single resonant point, which therefore resonates with greater amplitude. Because these resonant points commonly occur within the audible audio band, the rectangular capsules can theoretically produce flatter response, all else being equal. As SoundOnSound describes it, “the effect of the resonance is distributed and thereby diminished.”

Another is that the width of the capsule’s pickup field differs with the length of the membrane. In essence, a rectangular capsule mounted vertically is less sensitive to off-axis sound coming from above and below. This can aid the user in eliminating undesirable off-axis sounds.